Ink-jet printing is a system in which the ink is discharged from a nozzle and adhered to a printing material. This printing system can achieve excellent printing even on a curved, uneven or irregular surface because the nozzle is not in contact with the printing material. Accordingly, it has been expected that such a printing system would be applied to various fields.
In recent years, with regard to inks used in the ink-jet printing system, aqueous inks including water as a main component, oil-based inks including an organic solvent as a main component, and active energy ray-curable inks including a main component of a compound curable with active energy ray irradiation such as ultraviolet- or electron ray-irradiation have been provided. Especially, such an active energy ray-curable ink has been emphasized in certain fields of industry in terms of properties such as water-resistance, solvent-resistance and abrasion-resistance, or characteristics in which the ink can be applied to ink-nonabsorbable materials such as glass, metal, and plastics. Furthermore, among the active energy ray-curable inks, non-solvent ink substantially including no organic solvents has been best favored because such ink includes no volatile components, the drying step is therefore unnecessary, and is free from hazardous volatile organic components.
With regard to active energy ray-curable ink-jet printing inks that are colored with pigments or dyes, various compounds such as α-aminoketone-based, acyl phosphine oxide-based, hydroxyketone-based or thioxanthone-based compounds have been used as a photo-polymerization initiator in order to alleviate the cure-inhibiting phenomenon caused from the blockage of the active energy ray with the pigments incorporated therein. In particular, photo-polymerization initiators having an optical-absorption property of relatively longer wavelength are used therein. Even if the printed coating film is much thick, the use of such photo-polymerization initiators can alleviate the blockage problems caused from the reflection, absorption or the like of the active energy rays owing to the pigment incorporated therein, thereby achieve a deep curing of the printed coating film (See Patent Documents 1 and 2). In addition, the combination thereof has been alternatively employed (See Patent Documents 2 and 3).
However, most of the commercially available radical photo-polymerization initiators have a problem in which the generated radicals are trapped by oxygen in the air whereby the inhibition of the polymerization reaction is likely to occur. Therefore, there has been a demerit the curability is reduced particularly in a thin film regardless of the use thereof for the inks.
With regard to conventional printers employing flat printing, flexographic printing or the like, a low-concentration portion and high-concentration portion of a full-color print to a non-absorbable material are expressed with areas of the printed dots, and no large difference in the thickness of the printed coat is present. On the contrary, with regard to the image formation in the ink-jet printing system, the density of color is controlled with the size of the droplet and the dot density, the thickness of the printed coat varies depending on the density of color, and the difference of the thickness between the low-concentration portion and high-concentration portion is larger. In particular, the difference of the thickness becomes larger when an ink including no unreactive organic solvent is used. Therefore, the ink coat in the portion of light color is thin in thickness, intervals between the dots tend to be wide, and the most of the ink portions adhered onto the printed surface is in contact with the air, and therefore, such portions are subjected to the phenomenon of inhibiting a polymerization owing to oxygen. Consequently, the formed image often has portions that are deficiently cured, and the occurrence of nonuniform curing is inevitable in a general image having a distribution of color density.
Furthermore, among commercially available photo-polymerization initiators, easily obtainable α-aminoketone-based or acyl phosphine oxide-based photo-polymerization initiators have low solubility with respect to compounds having ethylenic double bonds, which make it possible to attain the active energy ray-curability. Therefore, particularly in a solventless ink, such a photo-polymerization initiator causes precipitation therein when the amount of the initiator incorporated therein is increased in order to improve the curability, the nozzle is then clogged with the precipitation, and this interferes with the discharging of the ink.
Therefore, the shortage of the curability cannot be improved by increasing the amount of the initiator added thereto, and the conventional active energy ray-curable ink-jet printing inks cannot achieve curability sufficient for practical use especially when the coating film is thin in thickness.
In addition, white inks in which a white pigment such as titanium oxide is used as a coloring agent necessitate a much higher concentration of the coloring agent compared with the other colors in order to obtain sufficient concealment, and the effect of the concealment owing to the coloring agent significantly increases, thereby further deteriorating the curability. For example, when the large amount of a photo-polymerization initiator having an optical absorption in a range of the longer wavelengths is used therein, there has been a problem in which the cured product turns yellow.
On the other hand, an α-hydroroxyketone-based photo-polymerization initiator which does not have an optical absorption in a range of the longer wavelengths as the above-described photo-polymerization initiator has, does not cause the yellow-coloring problem, and has excellent curability, has been disclosed (See Patent Documents 4 to 6).
However, when the photo-polymerization initiators disclosed in these documents are singularly used in an active energy ray-curable ink-jet printing ink, sufficient curability cannot be obtained particularly in curing a thin film. Thus, a photo-polymerization initiator which can impart sufficient curability to the formed coating film has been long expected.
The reason such a photo-polymerization initiator could not be achieved is because the curability has been conventionally confirmed by often using a coating film about 10 μm thick, and a photo-polymerization initiator has not been developed by studying its chemical composition and combination so as to simultaneously impart curability to coating films in a wide range of thickness, having a thickness of several micrometers or much larger thickness.
Patent Document 1: Japanese Examined Patent Application, Publication No. H6-21256
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2001-525479 (Japanese translation of PCT international application)
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2002-241647
Patent Document 4: PCT International Publication No. WO 03/040076
Patent Document 5: PCT International Publication No. WO 2004/009651
Patent Document 6: PCT International Publication No. WO 2004/092287